Petrogenesis of the amphibole-rich veins from the Lherz orogenic Iherzolite massif (Eastern Pyrenees, France): a case study for the origin of orthopyroxene-bearing amphibole pyroxenites in the lithospheric mantle

The Lherz orogenic Iherzolite massif (Eastern French Pyrenees) displays one of the best exposures of subcontinental lithospheric mantle containing vei ns of amphibole pyroxenites and hornblendites. A reappraisal of the petroge nesis of these rocks has been attempted from a comprehensive study of their mutual structural relationships, their petrography and their mineral compo sitions. Amphibole pyroxenites comprise clinopyroxene, orthopyroxene and sp inel as early cumulus phases, with garnet and late-magmatic K2O-poor pargas ite replacing clinopyroxene, and subsolidus exsolution products (olivine, s pinel II, garnet II, plagioclase). The original magmatic mineralogy and roc k compositions were partly obscured by late-intrusive hornblendites and ove r a few centimetres by vein-wallrock exchange reactions which continued dow n to subsolidus temperatures for Mg-Fe. Thermobarometric data and liquidus parageneses indicate that amphibole pyroxenites started to crystallize at P greater than or equal to 13 kbar and recrystallized at P < 12 kbar. The hi gh Al-VI/Al-IV ratio (> 1) of clinopyroxenes, the early precipitation of or thopyroxene and the late-magmatic amphibole are arguments for parental melt s richer in silica but poorer in water than alkali basalts. Their modelled major element compositions are similar to transitional alkali basalt with a bout 1-3 wt% H2O. In contrast to amphibole pyroxenites, hornblendites only show kaersutite as liquidus phase, and phlogopite as intercumulus phase. Th ey are interpreted as crystalline segregates from primary basanitic magmas (mg = 0.6; 4-6 wt% H2O). These latter cannot be related to the parental liq uids of amphibole pyroxenites by a fractional crystallization process. Rath er, basanitic liquids mostly reused pre-existing pyroxenite vein conduits a t a higher structural level (P less than or equal to 10 kbar).A continuous process of redox melting and/or alkali melt/peridotite interaction in a vei ned lithospheric mantle is proposed to account for the origin of the Lherz hydrous veins. The transitional basalt composition is interpreted in terms of extensive dissolution of olivine and orthopyroxene from wallrock peridot ite by alkaline melts produced at the mechanical boundary layer/thermal bou ndary layer transition (about 45-50 km deep). Continuous fluid ingress allo wed remelting of the deeper veined mantle to produce the basanitic, strongl y volatiles enriched, melts that precipitated hornblendites. A similar mode l could be valid for the few orthopyroxene-rich hydrous pyroxenites describ ed in basalt-hosted mantle xenoliths.